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Quels mécanismes physiologiques entrent en jeu après la fermeture stomatique ? : Etudes des pertes d'eau résiduelles des feuilles lors de sécheresses sévères et implications pour la survie des plantes

Year: 2025
Type: dissertation
Abstract: Climate change is increasing the frequency and intensity of extreme weather events, exerting increasing pressure on vegetation. Among these, drought disrupts hydrological cycles and the plant hydraulic system, thereby compromising growth, productivity and even survival. Recent advances in plant hydraulics have revealed a close relationship between drought-induced mortality and xylem resistance to embolism. However, it remains difficult to predict the survival time of a plant subjected to prolonged drought that could lead to fatal hydraulic failure. Indeed, most studies have focused on the role of stomata in regulating water loss during drought, neglecting the residual water losses that persist after stomatal closure during prolonged and/or severe droughts. Yet, these water losses appear crucial for predicting survival, as they define the residual leaf conductance and the associated trait, the minimal leaf conductance. In this context, this thesis explores the mechanisms of residual water loss in leaves under severe drought in a crop, sunflower (Helianthus annuus), and a tree species, tulip poplar (Liriodendron tulipifera). Conducted along a gradient of water stress, this work is structured around three main axes and aimed at better understanding the determinants of minimal leaf conductance and their impact on plant survival : (i) analysis of stomatal leakiness, (ii) the significance of spatial stomatal coordination and its role in balancing transpiration and carbon assimilation, and (iii) the response of different cuticular layers. Using complementary and independent techniques, we demonstrated the occurrence of stomatal leakage from the onset of stomatal closure (corresponding to turgor loss) to high levels of water stress (ΨP50), which strongly influences residual leaf conductance and thus survival. Our results also highlight that the heterogeneous closure of stomata, referred to as stomatal patchiness, represents a drought response. In particular, between turgor loss (Ψtlp) and the onset of drought-induced photosynthetic decline (ΨP12), this spatial heterogeneity acts as a buffering mechanism that extends leaf physiological activity under water stress. Furthermore, by analysing this phenomenon across two temporal scales, we reveal a spatial hierarchy in patchiness, with small patches aggregating into larger ones revealing a spatial hierarchy that improves the leaf’s response to environmental cues. In parallel with stomatal regulation, the cuticle also plays a key role in limiting water loss. Analysis of its different layers under water stress reveals that they do not have the same influence on limiting water loss during drought. While the amounts of cutin and waxes both increase with drought intensity, only cutin effectively contributes to reducing water loss, delaying the onset of hydraulic failure and, consequently, mortality. In conclusion, these results highlight the importance of both stomatal and cuticular mechanisms in regulating water loss during severe drought and their impact on plant survival. The combination of spatially heterogeneous stomatal responses and the differentiated contributions of cuticular layers illustrates how leaves attempt to conserve water. This coupling between stomatal and cuticular flexibility reflects an efficient adaptive strategy aimed at maintaining physiological functions while minimising water loss.
Language: English
Open Access status: green
OpenAlex ID: W7133571379
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